ABSTRACT Physical inactivity in the United States has become a critical public health concern for all age groups. With a large segment of the population rapidly aging, there is growing interest in ensuring mid to older age adults (50+) stay active and reduce sedentariness, to decrease their morbidity and mortality risk. While older adults are advised to engage in at least 150 minutes/week of moderate to vigorous physical activity (PA), most do not. Specific theory based behavioral strategies have been shown to positively affect health and activity behaviors. There is also emerging evidence that mindfulness based interventions (MBI) may facilitate PA through increased mind-body awareness and self-regulation. However, research for MBIs and PA is currently limited, both in quantity and methodological rigor. The broad long-term objective is to optimize physical and sedentary activity levels in middle-older age adults through an integrated mindfulness and behavioral approach scalable for broad dissemination. The University of Minnesota (UMN) will work with the YMCA Greater Twin Cities, a community based organization committed to providing accessible resources to ?build healthy spirit, mind and body for all? and making healthy aging a priority. To decrease time between discovery and translation, a two-phase hybrid effectiveness-implementation design will be used. Phase I (R21) will address the following: Aim 1 will focus on project organization, barrier/facilitator assessment, protocol development, and training. Aim 2 will test project protocols at multiple levels in a randomized pilot study (n=30) of Mindful Movement versus a modified 10 Keys to Healthy Aging educational control using transition milestones. Phase II (R33) will address: Aim 3 to determine the relative effectiveness of Mindful Movement versus control in a full-scale randomized controlled trial (RCT, n=182) using accelerometer based PA at 12 weeks (primary outcome) and at 26 and 52 weeks (secondary outcomes). Other secondary measures include sedentary activity, quality of life, exercise self-efficacy and expectations, mindfulness, wellbeing, bodily pain, and adverse events. Aim 4 will facilitate RCT interpretation by providing contextual data from participants, staff, and organizational leadership. These will include: enrollment, adherence, follow up, and fidelity rates; and perceptions of barriers/facilitators, relevance, practicality, affordability and acceptability. This information will be used to create a toolkit for implementation at other YMCA sites. This project is innovative in that it will be among the first to assess MBIs for enhancing older adults' PA in community based settings using a design intended to facilitate and accelerate research translation. Through the UMN and YMCA partnership, there is great potential to develop a sustainable and replicable PA intervention with meaningful impact and nationwide reach.